Power Curves of Tribovoltaic Systems: Set-up, Estimation and Analyses

In recent years, TriboElectric NanoGenerators (TENGs) have become a prominent research topic, driven by the increasing energy demands and the need for renewable and self-sustaining power sources. They are devices capable of harvesting and converting mechanical energy into usable electrical power exploiting the triboelectric effect. The main limitation of TENGs concerns the fact that the produced current is very low (in the order of nA), and is alternating, needing a rectifier in the related device. For these reasons, an increasing attention has been paid to the tribovoltaic effect, a similar phenomenon which appears in p-n or Schottky junctions. The tribovoltaic effect consists in the generation of charges when the two materials slide against each other (bindington effect). The built-in electric field at the interface separates and drives the electron–hole pairs, generating a continuous electric current in the order of μA. Considering the limited amount of literature on this topic, especially concerning macro-scale systems, it is strategic to give more insights and deepen all the related aspects. The aim of the present thesis activity is to study in detail the properties of different tribovoltaic systems. In particular, the current and, consequently, the generated power, have been estimated for metallic balls sliding over a semiconducting flat surface in a ball-on-flat configuration. At first, the implementation and optimization of the laboratory apparatus has been carried out, followed by systematic analyses on two different sliding pairs: a W C − Co ball sliding over a Si substrate and a W C − Co ball over a DLC film grown on a steel substrate. The pristine set-up is a CETR-UMT-3 tribometer, settled in a reciprocating ball-on-flat configuration, able to vary the applied load, velocity and humidity between the sliding surfaces. The electrical data have been acquired using a Keithley 2612A source-meter as measurement unit. As the source-meter had not a graphical working interface directly available on a computer, a significant part of the thesis activity consisted in writing specific codes to control the instrument. The systematic analysis on the B−doped Si/W C − Co pair lead to the estimation of a generated current of some tens of μA, showing no dependence on the sliding velocity. The corresponding power is estimated to be some nW , and it is found to be constant with increasing velocity. For the DLC/W C − Co coupling, a reliable statistical analysis for the short circuit current can not be performed due to the significant variability observed among different samples, which exhibited different behaviours under otherwise identical experimental conditions. The power estimation showed that the generated power remained constant at 0.06 · 10^−9 W independently on the velocity. In addition to the electrical analysis, also chemical analyses using Energy Dispersive X-ray spectroscopy coupled with an Environmental Scanning Electron Microscope and Profilometry have been performed in order to find the influence of wear-related phenomena. For both sliding pairs, material transfer from the substrate to the ball is observed; however, this does not result in the formation of a stable tribolayer. The friction coefficient and the wear rate of the sliding pairs have been estimated and correlated to the electrical analysis.